Store attrs with a large number of values separately from the
main entry blob. Note - we need support for large DUPSORT values
for this to be generally usable.
If an add or modify op referenced a previously unused attrtype,
the mi_ads/mi_numads would be incremented to include it. If the
op fails to commit, these additions must also be reverted.
Since MDB writes are fully serialized in txn_begin, we can use CSNs as
commit sequence numbers, not just change sequence numbers. This will
prevent most instances of out-of-order writes, so syncprov will almost
never need to transmit a change without a cookieCSN. (Out-of-order is
still possible, if the write was received via syncrepl.)
